Anaerobic compositions are stable and remain in a liquid, unpolymerized state in the presence of oxygen, but cure to a polymerized, solid state in the absence of oxygen. Such compositions are generally characterized as being one-part systems, with the ability to cure at room temperature through a free radical reaction, and to cure through a reaction initiated on a substrate surface. The components of such compositions are often monomers, initiators, catalysts, stabilizers/inhibitors, and modifiers, which may be combined, typically by mixing, to provide formulations of various viscosities, colors, strengths, and adhesion properties.
As a result of their ability to cure in a substantially oxygen-free environment, anaerobic compositions have found great utility as adhesives, sealants, and bonding materials for use with closely mated surfaces, such as between interfitting metal parts. Known industrial uses include locking threaded fasteners, sealing threaded pipe connections, retaining cylindrical machine components, sealing flange joints, bonding structural components, and sealing porous metal castings. Anaerobic adhesives are typically packaged, sold and stored in containers which are permeable to oxygen, such as those made of polyethylene.
Cure of anaerobic compositions is usually accelerated by the presence of a suitable metal, such as a transition metal (e.g., iron, copper, and nickel), which renders such metals well-suited for bonding with such compositions. However, such metals are susceptible to corrosion, often times rendering them poorly suited for industrial applications where corrosion compromises the integrity of the parts.
In contrast, magnesium alloys, typically alloys which include magnesium, aluminum, zinc and manganese, are passive substrates which are characterized as having high resistance to corrosion and a high strength-to-weight ratio. Additionally, they are easily molded and have the ability to withstand high tolerances and high temperatures. As a result of these properties, magnesium alloys are finding utility in many commercial applications, such as automotive applications.
Magnesium alloys have traditionally been joined by riveting or welding. It is desirable, however, to bond magnesium alloys using adhesives, as this offers advantages in weight saving, fatigue strength, and corrosion resistance. Corrosion resistance is due in large part to the very low level or iron, copper, and nickel found in most common alloys. The lower quantity of these transition metals present in an alloy, the slower the rate of corrosion. However, the low level of these transition metals renders magnesium alloys particularly insensitive to anaerobic cure.
Iron and copper have been found to accelerate cure of anaerobic adhesives and typical anaerobic formulations generally require trace amounts of these elements to be present to effect cure. These elements are generally applied to the substrate surface as a primer prior to application of the anaerobic adhesive. Adhesive systems using such primers are known as two-part systems.
U.S. Pat. No. 5,411,998 to McArdle et al. discloses the use of anaerobic compositions in one and two part systems for sealing surfaces having thick bondlines. The one-part systems include as a cure system the combination of saccharin and an onium salt. Example 18 of McArdle also discloses a two-part system, with each part having a cure system, the first part including cumene hydroperoxide, saccharin and an onium salt, and a second part which includes saccharin and acetylphenyl hydrazine. Cure is achieved when the two parts are mixed together in equal amounts.
The additional step of treating the surface of the substrate with a primer is undesirable as compared with one-part systems not requiring a primer as the need for a primer renders the adhesive more expensive, more labor intensive to apply, and more difficult to package and distribute. Therefore, there exists a need for a method of bonding magnesium alloy substrates and related passive substrates using one-part anaerobic adhesive systems. The present invention provides a solution to meet this need.